(1) Field of the Invention
The present invention relates to a blade with adaptive twisting and to a rotor provided with such a blade, and more particularly but not exclusively to a blade of a rotorcraft lift rotor.
(2) Description of Related Art
Conventionally, a blade extends longitudinally from a first end for fastening to a rotary hub of a rotor towards a second end that is referred to as a free end. Relative to the rotor, it can be understood that the blade extends radially from the first end to the second end.
Furthermore, the blade extends transversely from a leading edge to a trailing edge. The blade comprises in particular an outer covering that is provided with a first skin on its suction side, referred to for convenience as its “suction side skin”, and a second skin on its pressure side referred to for convenience as its “pressure side skin”.
A blade of a main lift rotor of a rotorcraft exerts lift during the rotary motion of said main rotor that enables the rotorcraft to be lifted and possibly also to be propelled. As a function of the pitch angle of the blade, the amount of lift that is developed can be made to increase or decrease. The aerodynamic angle of incidence of each aerodynamic profile of the blade, referred to as a “profile” for convenience, in a section normal to the pitch variation axis of the blade depends on the pitch angle of the blade. In contrast, it is found that from a threshold angle of incidence for a given profile, and thus for a given blade section, streams of air separate from the leading edge or the trailing edge of the profile. If the separation propagates and remains over a zone lying between two profiles that define a critical area along the span of the blade, this phenomenon causes the blade to stall, i.e. causes its lift to drop suddenly. Furthermore, air stream operation gives rise to turbulence that is the origin of an increase in the drag coefficient of the blade and to an increase in vibration.
In order to limit separation, one solution consists in giving the blade a geometrical twist. It should be observed that the geometrical twist of a blade may be defined by the angle formed between the chord of the profile of each blade section relative to a reference plane for the blade. Sometimes, each blade profile presents twist relative to the pitch variation axis of the blade at an angle that is identified relative to such a reference plane.
For a given blade trajectory, it can be understood that twist has a direct influence on the aerodynamic angle of incidence of each profile. Under such conditions, the term “twist relationship” designates the way in which said twist angles vary along the span of the blade.
The twist relationship of a blade does not vary. The twist relationship results from accepting a compromise that ensures the rotor operates properly over the entire flight domain.
It is found that a small amplitude of twist over the entire span of the blade helps minimize the power consumed by the lift rotor of a rotorcraft in forward flight. However, a large amplitude of twist over the entire span of the blade serves to minimize the power consumed by the lift rotor of a rotorcraft while hovering, but remains unacceptable during forward flight. It should be observed that the term “small amplitude” means an amplitude of less than 6 degrees, for example, whereas a “large amplitude” means an amplitude of more than 20 degrees, for example.
Thus, an amplitude of twist lying between the above small and large amplitudes represents a power consumption compromise between a stage of forward flight and a stage of hovering flight.
In order to avoid such a compromise, proposals have been made to modify the twist of a blade actively, at least locally.
In a first solution, at least one flap is used that locally extends the trailing edge of the blade. By modifying the angle at which the flap is deflected relative to the blade, the local geometry of the blade is modified together with the corresponding aerodynamic characteristics of its profile.
That first solution presents the advantage of giving rise to significant deformation and twisting. However, that first solution involves adding a flap, thereby giving rise to extra weight and mass remote from the center of gravity, and also giving rise to the presence of mechanical movements that need to be sustained and to turbulence being created at the side edges of the flaps.
The following publications relate to actuating such flaps:    O. Dieterich, B. Enenkl, D. Roth: Trailing edge flaps for active rotor control, Aeroeslastic characteristics of the ADASYS rotor system, American Helicopter Society, 62nd Annual Forum, Phoenix, Ariz., May 9-11, 2006,    S. R. Hall and E. F. Prechtl: Preliminary Testing of a Mach-Scaled Active Rotor Blade with a Trailing Edge Servo-Flap, Massachusetts Institute of Technology 77 Massachusetts Ave. Cambridge, Mass. 02139-4307 USA, 2000,    V. Giurgiutiu: Active-Materials Induced-Strain Actuation for Aeroelastic Vibration Control, The Shock and Vibration Digest, Vol. 32, No. 5, September 2000, 355-368.    F. K. Straub, D. K. Kennedy, D. B. Domzalski, A. A. Hassan, H. Ngo, V. Anand, and T. Birchette: Smart material-actuated Rotor Technology, Journal of intelligent Material Systems and Structures, Vol. 15 Apr. 2004,    C. K. Maucher, B. A. Grohmann, P. Jänker, A. Altmikus, F. Jensen, H. Baier: Actuator design for the active trailing edge of a helicopter rotor blade,    K. Thanasis: Smart Rotor Blades and Rotor Control for Wind Turbines, State of the Art, UpWind internal report for WP 1B3, December 2006.
Likewise, the following documents U.S. Pat. No. 7,424,988, US 2008/0237395, U.S. Pat. No. 6,513,762, U.S. Pat. No. 5,387,083, WO 00/41501, and WO 96/01503 all mention the presence of flaps.
In a second solution, piezoelectric fibers, piezo-composite patches, or indeed composite macro-fibers are used in the suction side and/or pressure side skin in order to generate twist of the blade. That second solution is described in particular in the following publications:    D. Thakkar, R. Ganguli: Induced shear actuation of helicopter rotor blade for active twist control, Thin-Walled Structures 45 (2007) 111-121.    J. P. Rodgers, N. W. Hagood: Design, manufacture, and testing of an integral twist-actuated rotor blade, 8th International Conference on Adaptive Structures and Technology, Wakayama, Japan, 1997.    J. Riemenschneider, S. Keye, P. Wierach, H. Mercier des Rochettes: Overview of the common DLR/ONERA project “active twist blade” (ATB), 30th European Rotorcraft Forum; 14.-16. Sep. 2004; Marseilles, France.    A. Kovalovs, E. Barkanov, S. Gluhihs: Active twist of model rotor blades with D-spar design, Transport—2007, Vol XXII, No 1, 38-44.    G. L. Ghiringhelli, P. Masarati, P. Mantegazza: Analysis of an actively twisted rotor by multibody global modeling, Composite Structures 52 (2001) 113-122.
Patent US 2007/0205332 uses an equivalent technique.
In a third solution, the suction side and the pressure side of the blade are made using skins of composite materials that are anisotropic.
In accordance with the publication “M. D. Schliesman: Improved methods for measurement of extension-twist coupling in composite laminate, Aeronautics and Astronautics, Inc., 1999”, traction exerted on such a skin can generate twist of the skin.
According to the publication “S. Ozbay: Extension-twist coupling optimization in composite rotor blades, thesis presented to the Georgia Institute of Technology in May 2006”, use is made of a system of sliding masses for generating twist by twisting such skins.
In a fourth technique, use is made of actuators for twisting the blade. For example, U.S. Pat. No. 7,264,200 discloses using an actuator to move flaps arranged at the free second end of the blade.
The documents US 2006/0186269, WO 99/36313, and WO 2007/145718 implements actuators for deforming a structure.
Documents U.S. Pat. No. 7,037,076 and WO 98/30448 use actuators for causing the second end of the blade to turn.
Document U.S. Pat. No. 5,505,589 makes provision for a weight that is movable in order to generate twist.
According to document U.S. Pat. No. 5,150,864, a cable having shape memory is used and it is heated in order to deform the blade.
Consequently, it can be seen that the techniques that seek to twist a blade actively implement members that are dedicated to such twist, such as actuators, weights, or heater means. That results in an increase in the weight of the blade.
It should be observed that Document WO 2008/052677 presents a wind turbine blade provided with an elongate central box structure extending in a longitudinal direction and that twists as a function of the incident wind, that wind striking the box structure in a transverse direction that is substantially perpendicular to the longitudinal direction of the blade.
Document US 2006/0186263 describes means for adjusting the angle of a blade, and more precisely for controlling the pitch of the blade, but not the twist of the blade. For this purpose, use is made of means acting solely on the root of the blade and relying on the physical phenomenon of stiffening in torsion and/or of returning to a flat shape.
The twist relationship of the blade remains unchanged, the twist angle remaining constant all along its span.
Document FR 2 737 465 seeks to minimize noise and vibration by using a device that makes use of an auxiliary actuator and an outer covering that is anisotropic.
Those documents do not form part of the technical field of the invention, i.e. the field of means for twisting a rotor blade enabling the lift of the blade to be optimized, in particular a rotorcraft blade. They are mentioned herein as technological background.
Furthermore, the state of the art includes the following documents WO 96/11337, GB 2 216 606, and EP 0 351 577.